In conventional dental handpieces, in particular those provided with an air turbine as shown in FIG. 4 having rotor 16 which rotatively drives burr sleeve 13, the burr sleeve 13 is rotatably supported by upper and lower ball bearings 14, 15. In some types of ball bearings, a plurality of balls 14c, 15c between inner and outer rings 14a, 15a, 14b, 15b of the bearings are supported by rolling bearing cages 17, 18 at regular circumferential intervals.
In these types of ball bearings, however, the cages 17, 18 supporting the balls 14c, 15c are shaken up and down due to the rotation of the burr sleeve. Thus, the cages 17, 18 are kept in unstable contact with the balls 14c, 15c, which causes wearing and damaging of both the balls and the cages, as well as shaking and noise of the overall handpiece. In particular, if a rolling bearing cage of crown type having a plurality of pockets with an opening for supporting the balls therein is used, the area supporting the balls in the pockets is smaller near the openings than other portions, and thus wearing of the cage is localized near the openings, leading to premature termination of the service life of the cage. Although the cages are pressed axially outwardly to some extent by the air stream flowing between the inner and outer rings of the bearings, the cages are not kept in sufficiently stable contact with the balls in the bearings.
Some of the conventional dental handpieces are further provided with dust preventive members 21, 22 projecting from and rotating with the burr sleeve 13 for preventing debris, saliva, blood, or the like from entering the interior of the head housing 11. The dust preventive members define on their outer surfaces gaps A which communicate with the rotation space of the rotor 16 via the upper or lower ball bearings 14, 15 at one end, and with outside the head housing 11 at the other end. Rotation of the dust preventive members 21, 22 generates high pressure areas in the gaps between the rotation space of the rotor 16 and outside the head housing 11, to thereby remarkably block the air stream out of the head housing 11. Consequently, even when the rotation of the rotor is stopped to establish negative pressure in the head housing 11, debris, saliva, blood, or the like will not be sucked into the interior of the head housing 11.
However, blocking of the air stream out of the head housing 11 will cause reduced flow of the air stream between the inner and outer rings of the ball bearings 14, 15. Therefore, the axially outward pressure exerted on the cages 17, 18 becomes still less sufficient for stabilizing the cages 17, 18, compared to the former type of the handpiece without the dust preventive members, and thus the cages are kept in less stable contact with the balls.